1. Field of the Invention
The present invention relates to a solid state image sensor, and more particularly, to improvement of an AMI (amplified MOS intelligent) type image sensor.
2. Description of the Background Art
Reflecting the increase in resolution of the horizontal direction and increase in density of recent image sensors, the area of one pixel is minimized in an image sensor, whereby the amount of incident light per 1 pixel is reduced. As a result, the intensity of a signal read out from an image sensor is decreased to lead to reduction of the S/N ratio. It is considered desirable to use an AMI type image sensor to overcome such problems.
FIG. 33 is a circuit diagram of a typical conventional AMI type image sensor. Referring to FIG. 23, an image sensor includes a photoelectric conversion element 31 formed by a PN junction, a MOS transistor 32 for amplification, a vertical-selection MOS transistor 33, an MOS transistor 34 for resetting photoelectric conversion element 31, a horizontal power supply line 35 for supplying power to a pixel including functional elements 31-34, a vertical-selection line 36 for selecting pixels disposed in the vertical direction, a vertical signal line 37 disposed in the vertical direction, a horizontal-selection MOS transistor 38 for selecting pixels arranged in the horizontal direction, a horizontal signal line 39, an I/V conversion amplifier 40 for converting signal current into voltage, a horizontal scanning circuit 41, and a vertical scanning circuit 42.
FIG. 34 is a circuit diagram for describing the operation of one arbitrary pixel in the image sensor of FIG. 33, and FIG. 35 is a timing chart for describing the operation of the pixel of FIG. 34. The signals in respective positions represented by various reference characters in FIG. 34 are denoted with the same reference characters in FIG. 35. In FIG. 35, a period 1H is one horizontal period in a general television system, a period H-BLK is a horizontal blanking period, and a period Read-Out corresponds to a signal reading period. Clocks V1 and H1 schematically represent clocks supplied to vertical scanning circuit 42 and horizontal scanning circuit 41, respectively.
At time T0, vertical-selection line 36 (VS) and horizontal power supply line 35 (VL) attain a high level, and transistors 32 and 33 are conductive. Since the output of photoelectric conversion element 31 is connected to the gate electrode of transistor 32, transistor 32 is conductive at an impedance depending upon an output potential V.sub.pd of photoelectric conversion element 31. At time T1 during a horizontal readout period Read-Out when signal Hi attains a high level to cause horizontal-selection MOS transistor 38 to attain a conductive state, vertical signal line 37 is electrically connected to I/V conversion amplifier, whereby a signal current corresponding to potential V.sub.pd of photoelectric conversion element 31 is read out as a voltage signal. At time T2 during the next horizontal period, vertical-selection line 45 (VR) of the subsequent row attains a high level, whereby photoelectric conversion element 31 is reset to a voltage level of Vreset supplied from horizontal power supply line 35 via MOS transistor 34. At time T3 of the next horizontal period, photoelectric conversion element 31 enters a storing mode that integrates a signal charge generated depending on incident light.
In a general interlace scanning operation of an image sensor, a first pixel group of alternate lines is scanned in a field A and a second pixel group of the other alternate lines is scanned in a field B.
In a conventional image sensor, reset of photoelectric conversion element 31 is carried out by vertical-selection line 36 of a subsequent row as shown in FIG. 33 in order to reduce the number of wirings disposed in a pixel array and to increase the integration density. In the image sensor of FIG. 33, the second pixel group will be reset during the scanning operation of the first pixel group in reading out field A, so that the signal charge in the second pixel group cannot be used. Similarly, the first pixel group will be reset during the scanning operation of the second pixel group in reading out field B, so that the signal charge of the first pixel group cannot be used. This means that the two-line-combined readout realized in a CCD image sensor to improve signal intensity is not possible in the AMI type image sensor of FIG. 33.